1. Field of the Invention
The present invention relates to a probing method and prober for testing the electrical characteristics of an electrical circuit device. More specifically, the present invention relates to a probing method and prober which can detect the positions of the surfaces of the electrodes of an individual electrical circuit device formed on a test target object (e.g., a wafer W).
2. Description of the Related Art
FIGS. 9A and 9B show an example of a prober for integrated circuits formed on a semiconductor wafer. A prober 10 has a loader chamber 1 from which a wafer W is to be transported, and a prober chamber 2 for testing the electrical characteristics of the wafer W transported from the loader chamber 1. The loader chamber 1 has a cassette support portion 3 where a cassette C for storing the wafer W is placed, a transfer mechanism (fork) 4 for transporting the wafer W to the loader chamber 1, and a subchuck 5 for prealigning the wafer W on the way when the fork 4 transports the wafer W. The prober chamber 2 has a wafer stage (to be referred to as “main chuck” hereinafter) 6, a mechanism for aligning the wafer W on the main chuck 6 (to be referred to as “alignment mechanism” hereinafter) 7, and a probe card 8. The prealigned wafer W is placed on the main chuck 6 with the fork 4. The main chuck 6 can move in the X, Y, Z, and θ directions. The electrodes of a plurality of is integrated circuits formed on the wafer W placed on the main chuck 6 are aligned with the probes of the probe card by the alignment mechanism 7 and the movement of the main chuck 6. The probe card 8 is fixed to a head plate 2A which forms the upper surface of the prober chamber 2.
As shown in FIGS. 9A and 9B, the alignment mechanism 7 includes a lower CCD camera 7A and upper CCD camera 7B. The two cameras are driven under the control of a control unit. The lower CCD camera 7A is provided to the main chuck 6. The lower CCD camera 7A senses probes 8A of the probe card 8 from lower side. The upper CCD camera 7B formed at the center of an alignment bridge 7C senses the wafer W on the main chuck 6 from upper side. The sensed images of probe 8A and wafer W are displayed on a monitor screen 9A of a display unit 9. The alignment bridge 7C moves in the upper portion of the prober chamber 2 and along guide rails 7D running in the Y direction from rear end (upper portion in FIG. 9B) of the prober chamber 2 to the probe center. The main chuck 6 has a target 7E which can move forward and backward to above the lower CCD camera 7A. The heights of the needle points of the probes 8A are obtained by sensing the needle points with the lower CCD camera 7A. The optical axes of the lower and upper CCD cameras 7A and 7B are set to coincide with each other through the target 7E. The level of the main chuck 6 at this moment is used as a reference level when performing alignment of the wafer W and probes 8A.
A test head T is rotatably provided to the prober chamber 2. The test head T is electrically connected to the probe card 8 through an interface (not shown). A signal from a tester is transmitted to the electrode pads of the wafer through the test head T and probes 8A. The electrical characteristics of the plurality of integrated circuits (chips) formed on the wafer W are measured from this signal.
To bring the wafer W and probes 8A into contact with each other with a predetermined pressure, the surface height of the wafer W is detected. For this detection, four portions of the wafer W equiangularly distant from each other in the circumferential direction, and the center of the wafer W, i.e., five portions, are sensed by the upper CCD camera 7B while the main chuck 6 is moved in the X and Y directions. The level in the Z direction of the main chuck 6 at each sensed position is obtained as the surface height of the wafer W. The average value of the obtained heights is calculated, thus detecting the surface height of the wafer W.
Conventionally, the heights of the positions of a plurality of portions on the wafer W are detected by focusing the upper CCD camera 7B on the surface of the wafer W. In this detection process, the main chuck 6 is vertically moved toward the focal point of the upper CCD camera 7B, so that the upper CCD camera 7B is focused on the surface of the wafer W. Due to this complicated operation, focusing takes time. As the surface of the wafer W has steps, the average value of the heights of the plurality of points on the surface of the wafer W does not necessarily coincide with the surface height of the individual chip formed on the wafer W. Consequently, each probe comes into contact with different chips with different needle pressures.